This is a small Aqua-Terrarium, set-up in a Tropiquarium. used to replace a vivarium (which had dry gravel, a water bowl and some rocks and wood).
When the crabs were first introduced to the new set up they became a lot more active, settling in almost straight away. They also seem to be less aggressive and territorial in this set-up, as they could dig their own burrows. Some are even hand tame now.
The filter is a Fluval 2, resting on it's side in the water.
A small piece of piping runs from the Fluval outlet, under the gravel and sand, and out above the pebbles on the right hand side.
The heater is 50W set to 80-82F, and is alongside the filter, stuck by suckers to the bottom glass. (The heater must be kept clear from debris and gravel at all times!).
I have siliconed the slate barrier together, this ensures the sand cannot get into the water (which is a problem in this design, a previous 'un-glued' build had been dismantled by the crabs). I used very course gravel along the bottom, and up the outlet side. This allows a good flow of water through the substrate. This is covered by a 4"-5" layer of silicon sand. A course sand that, again, will allow free water flow throughout the substrate, but also allows the crabs to dig burrows.
The water is changed about once a week (50%), and the sand is cleaned about once a month (50% scooped off the top and washed).
The crabs' diet is supplemented with pieces of broken stem plants, which can survive in the water until eaten, and the occasional pieces of cockle or smelt. This keeps waste to a minimum, and seems to keep the crabs happy.